log4net.Ext.Mail

A MailKit-based SMTP appender for log4net, recommended as the replacement for the deprecated built-in System.Net.Mail.SmtpClient appender.

Overview

log4net.Ext.Mail provides a satellite package that extends log4net with a modern email appender. It uses MailKit instead of the deprecated System.Net.Mail.SmtpClient, so you can reliably send log events via SMTP on modern servers.

The appender exposes the same API as the legacy log4net.Appender.SmtpAppender, making migration straightforward - you only need to change the type attribute in your log4net configuration.

Installation

dotnet add package log4net.Ext.Mail

Quick Start

Configure the appender in your App.config or app.config:

<configuration>
  <configSections>
    <section name="log4net" type="System.Configuration.IgnoreSectionHandler" />
  </configSections>

  <log4net>
    <appender name="SmtpAppender" type="log4net.Ext.Mail.Appender.SmtpAppender, log4net.Ext.Mail">
      <to value="recipient@example.com" />
      <from value="sender@example.com" />
      <subject value="Log Event" />
      <smtpHost value="mail.example.com" />
      <port value="587" />
      <enableSsl value="true" />
      <authentication value="Basic" />
      <username value="user@example.com" />
      <password value="password" />
      <bufferSize value="512" />
      <lossy value="true" />
      <evaluator type="log4net.Core.LevelEvaluator">
        <threshold value="ERROR" />
      </evaluator>
      <layout type="log4net.Layout.PatternLayout">
        <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
      </layout>
    </appender>

    <root>
      <level value="INFO" />
      <appender-ref ref="SmtpAppender" />
    </root>
  </log4net>
</configuration>

Then configure log4net in your code:

[assembly: log4net.Config.XmlConfigurator(Watch = true)]

Key Differences from the Built-in Appender

Feature	Built-in	log4net.Ext.Mail.Appender.SmtpAppender
SMTP Client	System.Net.Mail.SmtpClient (deprecated)	MailKit
TLS Support	Limited	Full (Auto, Explicit, Implicit)
Modern Servers	Often fails	✓ Recommended
smtpHost	Optional	Required
enableSsl	true/false	true/false (negotiates Auto/None)
Ntlm auth	Uses Windows logon	Requires explicit credentials

Configuration Options

Required

• to - recipient email address (comma or semicolon delimited)
• from - sender email address
• subject - email subject line
• smtpHost - SMTP server hostname

Optional

• cc - carbon copy recipients
• bcc - blind carbon copy recipients
• replyTo - reply-to address
• port - SMTP port (default: 25)
• enableSsl - negotiate TLS/STARTTLS (true/false, default: false)
• authentication - None, Basic, or Ntlm (default: None)
• username - username for authentication
• password - password for authentication
• priority - email priority (Low, Normal, High)
• bufferSize - buffer up to N events before sending (default: 512)
• lossy - discard buffered events if trigger never fires (default: false)
• evaluator - custom evaluator to trigger sending (e.g., LevelEvaluator)

Frameworks

Targets .NET Standard 2.0, so it works with:

• .NET Framework 4.7.1+
• .NET 8+

Documentation

For complete configuration examples and detailed options, see the log4net SmtpAppender documentation.

License

Licensed under the Apache License 2.0. See the LICENSE file for details.